This invention relates to solid electrolyte valve-metal capacitors and more particularly to tantalum capacitors having a solid manganese dioxide electrolyte with graphite being buried in portions thereof.
It is well known that a solid electrolytic valve-metal capacitor may be manufactured by the following method. A sintered porous tantalum body is anodized formin a film of tantalum oxide over all its exposed surfaces. Several coatings, typically about eight, of a manganous nitrate solution are applied over the tantalum oxide including regions in the pores of the tantalum body. Each coating is fired at a temperature of about 400.degree. C, pyrolyzing and transforming the manganous nitrate to a semiconductor, namely manganese dioxide (MnO.sub.2). The composite coating of pyrolyzed MnO.sub.2 is then coated with a layer of graphite from an aqueous suspension, a layer of silver from a paint suspension, a layer of solder, and a metallic conductor. The body of the unit may then be encapsulated for example, in an epoxy resin.
Electronic components including solid electrolyte capacitors are frequently required to withstand 360.degree. C for several minutes, while they are being solder connected to printed wire boards or other circuit assemblies. In the above noted standard capacitor construction, failure at 360.degree. C typically occurs due to inadequacies in the silver, the solder, and the encapsulating resin. For example, the organic binder in the silver is subject to degradation and the organic vapors cause reduction of MnO.sub.2 to species of lower conductivity; the silver metal may also be leached by the covering layer of solder. The solder is applied with flux which is another source of organic vapors liable to cause reduction of MnO.sub.2. The solder itself is molten at 360.degree. C which is clearly an undesirable feature. The most severe problem of all is caused by vapors from the encapsulating resin. Also, encapsulation intensifies the reaction of any vapors from the binder or flux due to entrapment.
Attempts have been made to replace the silver/solder system with a sprayed metal system which is usually a high melting solder such as 95% lead, 5% tin. However this construction is more difficult to carry out and leads to lower yields. In addition, the sprayed solder is molten at 360.degree. C and solder droplets are often observed on the outside of the units after exposure. It also does nothing to alleviate the problem of vapors from the molding resin interacting with MnO.sub.2. For these reasons it is quite inadequate for 360.degree. C exposure although it has made possible units which may be exposed to 300.degree. C which is somewhat higher than the standard units will withstand.
A forthright but expensive solution to this problem has been to house the conventional capacitor without any organic material in a hermetically sealed metal can.
In a presently pending patent application, Ser. No. 540,028 filed Jan. 10, 1975, a capacitor is described that solves this problem in a more subtile manner. The counterelectrode consists of a base metal part that is buried in the composite MnO.sub.2 coating. Direct contact between the base metal and the easily reducible MnO.sub.2 is avoided by plating the base metal with a noble metal or by covering it with a graphite coating. The present invention provides an alternative and complementary solution to this problem.
It is therefore an object of the present invention to provide a novel counterelectrode system in a solid valve metal capacitor.
It is a further object of the present invention to provide a low cost solid valve-metal capacitor requiring no new steps in manufacturing.
It is a further object of this invention to provide a solid valve-metal capacitor having superior performance characteristics especially after exposure to elevated temperatures.
It is yet a further object of this invention to provide a solid valve-metal capacitor having a housing of an inexpensive organic material, that is capable of exposure to soldering processes involving temperatures as high as 360.degree. C for several minutes without degradation.